logtape.c: allocate read buffer even for an empty tape.

jeff-davis · jeff-davis · commit 8021985d7919 · 2020-02-19T10:04:17.000-08:00
Prior to this commit, the read buffer was allocated at the time the tape was rewound; but as an optimization, would not be allocated at all if the tape was empty. That optimization meant that it was valid to have a rewound tape with the buffer set to NULL, but only if a number of conditions were met and only if the API was used properly. After 7fdd919 refactored the code to support lazily-allocating the buffer, Coverity started complaining. The optimization for empty tapes doesn't seem important, so just allocate the buffer whether the tape has any data or not. Discussion: https://postgr.es/m/20351.1581868306%40sss.pgh.pa.us
diff --git a/src/backend/utils/sort/logtape.c b/src/backend/utils/sort/logtape.c
@@ -544,11 +544,8 @@ ltsConcatWorkerTapes(LogicalTapeSet *lts, TapeShare *shared,
 static void
 ltsInitReadBuffer(LogicalTapeSet *lts, LogicalTape *lt)
 {
-	if (lt->firstBlockNumber != -1L)
-	{
-		Assert(lt->buffer_size > 0);
-		lt->buffer = palloc(lt->buffer_size);
-	}
+	Assert(lt->buffer_size > 0);
+	lt->buffer = palloc(lt->buffer_size);
 
 	/* Read the first block, or reset if tape is empty */
 	lt->nextBlockNumber = lt->firstBlockNumber;
@@ -839,13 +836,10 @@ LogicalTapeRewindForRead(LogicalTapeSet *lts, int tapenum, size_t buffer_size)
 	/* Allocate a read buffer (unless the tape is empty) */
 	if (lt->buffer)
 		pfree(lt->buffer);
+
+	/* the buffer is lazily allocated, but set the size here */
 	lt->buffer = NULL;
-	lt->buffer_size = 0;
-	if (lt->firstBlockNumber != -1L)
-	{
-		/* the buffer is lazily allocated, but set the size here */
-		lt->buffer_size = buffer_size;
-	}
+	lt->buffer_size = buffer_size;
 }
 
 /*
